A spark plug for an internal combustion engine such as automotive engine includes a center electrode arranged in a combustion chamber of the internal combustion engine and a ground electrode facing the center electrode with a spark discharge gap left therebetween so as to generate a spark discharge in the spark discharge gap inside the combustion chamber of the internal combustion engine for ignition of fuel fed into the combustion chamber. The respective electrodes of the spark plug are made of high heat resistant material such as Ni-based alloy as each of these spark plug electrodes is placed in the combustion chamber of which the atmosphere reaches a high temperature.
In this spark plug, the ground electrode is generally joined by electric resistance welding to an end portion of a metal shell. However, the ground electrode may not be resistance welded to the metal shell with sufficient welding strength so that breakage or damage occurs in the welding joint between the ground electrode and the metal shell during adjustment of the spark discharge gap, after manufacturing of the spark plug or during use of the spark plug. For example, it is mentioned in paragraph 0046 of Japanese Laid-Open Patent Publication No. 2003-229231 as follows: “If the Vickers hardness of the ground electrode is 220 Hv or higher, the ground electrode is so poor in bending workability that separation or breakage occurs from the welding interface during bending. In this way, the ground electrode of high hardness cannot be properly joined to the housing by the conventional joining method due to the difficulty for the ground electrode to secure both of breakage resistance and bending workability.”
There have thus been made several proposals about the welding state and welding method of the metal shell and the ground electrode. For example, one proposal discloses a method of joining a ground electrode to a housing after subjecting the ground electrode to bending (see Japanese Laid-Open Patent Publication No. 2003-229231). Another proposal discloses a spark plug in which a ground electrode is embedded at an end thereof in a housing so as to increase welding area for improvement in welding strength (see Japanese Laid-Open Patent Publication No. 2005-251727). Still another proposal discloses a spark plug in which the dimensions of a metal shell or the setting angle of a ground electrode is controlled so as to secure welding strength (see Japanese Laid-Open Patent Publication No. 2005-228562).
As the spark plug generates a spark discharge in the spark discharge gap, the respective electrodes of the spark plug require not only heat resistance but also spark wear resistance. For example, a Ni-based alloy with a high Ni content is known as material for improvement in spark wear resistance. The ground electrode of such a Ni-based alloy is high in spark wear resistance, but is more difficult to weld than that of a Ni-based alloy with a not-so-high Ni content. It is thus necessary to take any measure, such as supplying a large electric current to the ground electrode and the metal shell, for electric resistance welding of the ground electrode and the metal shell. Even by taking the above measure, the ground electrode may not be joined to the metal shell by electric resistance welding with sufficient welding strength.
It is accordingly an object of the present invention to provide a spark plug having a ground electrode joined securely by electric resistance welding.